sh_eth: check platform data pointer

Check the platform data pointer before dereferencing it and error out of the
probe() method if it's NULL.

This has additional effect of preventing kernel oops with outdated platform data
containing zero PHY address instead (such as on SolutionEngine7710).

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>